1. Field of the Invention
The present invention relates to an injection molding method and an injection molding apparatus that are used to form a resin molded component such as a tailgate and a bumper.
2. Related Art
Most of resin components for a vehicle are manufactured by injection molding. A complex shaped product, a large-scale product etc. have a problem that a resin cannot be injected into the end portion of a metal mold cavity. Accordingly, various methods have been suggested.
In JP-A-2000-127203, there is disclosed a hot runner apparatus including a receiving runner portion which extends substantially in parallel to a fixed die plate, a deliver runner portion, a hot runner portion which has heating means, a groove portion which is formed in a metal mold and to which the deliver runner portion is accessible, and driving means for driving the hot runner portion. The hot runner apparatus can be commonly used in a plurality of metal molds by driving the hot runner portion to allow an outlet opening to come into contact with a sprue port at the time of molding and by driving the hot runner portion in a reverse direction to allow the delivery runner portion to pass through the groove portion at the time of replacing a stage so that the metal mold is separated from the hot runner portion.
In JP-A-2005-297384, there is disclosed an injection molding apparatus including a plurality of injection nozzles which inject a melted resin into a mold cavity and control means which is formed in each nozzle so as to instruct an injection ratio of the resin to be injected from each injection nozzle. The control means has a memory which stores the injection ratio corresponding to time elapsed from the start of injection. The injection ratio stored in the memory is set such that injection pressure of each injection nozzle is equivalent to each other.
In JP-A-09-099458, there is disclosed a technique for preventing inferior phenomenon arising when a molding product is manufactured by resonating a whole metal mold with an ultrasonic vibrator at a half of a wavelength by an ultrasonic wave or by providing a resonator resonated by a vibration in a movable metal mold so as to be resonated by an ultrasonic wave and to fill a resin at a high speed, so that a skin layer is reduced.
In an injection molding, a low injection pressure is essentially demanded in order to omit a post process such as a deburring process, to mold a large-scale component, to attain an unmanned system, to perform multiple processes, to reduce a cost, and the like.
However, in a low-pressure molding, when a pressure at the time of holding a pressure decreases, a contraction force during a cooling solidification is larger than a holding pressure force. As a result, an outer appearance at a cavity end portion is poor, where the cavity end portion is a position in which a pressure supply and a flow supply are not smoothly conducted. In order to prevent the poor outer appearance, the holding pressure is generally maintained at 30 MPa or more.
FIG. 1 is a graph showing a relation between a state (temperature) and a specific volume of a resin used in an experiment that is conducted by the inventors. As shown in the graph, when the temperature of the resin filled in the cavity is high, a contraction rate (specific volume) increases together. Further, a first inflection point T1 where a slope of the specific volume is steep and a second inflection point T2 where the slope of the specific volume is less steep are found during the time the melted resin is cooled.
In JP-A-2000-127203, the hot runner apparatus is commonly used in a plurality of metal molds by separating the hot runner apparatus from the metal mold. However, a position where a nozzle of the hot runner apparatus comes into contact with the metal mold is always the same. Accordingly, a resin receiving port needs to be provided at the same position even when a shape of a product is different. For this reason, when a gate is set depending on the shape of the product, a distance from the resin receiving port to the gate may increase. Thus, the position of the resin receiving port is not always suitable for the shape of the product.
In JP-A-2005-297384, a flow amount of the resin flowing in a distributed runner is calculated so that an injection rate corresponds to a case where injection pressures of injection nozzles are equal to each other. When the resin is injected at such an injection rate, the injection pressures of the injection nozzles become equal to each other. Accordingly, the pressure in the cavity can be lowered. However, a temperature control is not mentioned. As a result, it is not possible to predict a reduction of a contraction amount in the case of conducting an injection molding of a large-sized molded product.
In JP-A-09-099458, when an injection is conducted at the injection rate of 150 cm3/sec or more by using an ultrasonic vibrator capable of generating a vibration frequency in the range of 1 kHz to 10 kHz, a flow of the resin is enhanced. However, an apparatus for vibrating a whole metal mold of a large-sized component by using the ultrasonic vibrator capable of generating a vibration frequency in the range of 1 kHz to 10 kHz is large in size. As a result, it is not suitable to conduct an injection molding of, for example, a large-sized component of a vehicle.
In addition, as described above, a decrease in pressure of an injection molding method has been demanded. However, when the pressure at the time of holding a pressure is lowered, a deformation of a product arises. Likewise, the methods disclosed in the above documents cannot solve such a problem that the deformation of the product arises when the pressure at the time of holding a pressure is lowered.